The present invention generally relates to gas turbine engines and, more particularly, to cooled components, such as combustor liners.
Customers and regulation agencies are continually applying pressure on engine manufactures to achieve lower specific fuel consumptions (SFCs) and emissions. This in turn requires combustors to operate at hotter temperatures yet reduce the cooling air required to cool the walls. There are two highly effective methods of cooling combustor walls, effusion (known by other names such as multi-hole) and impingement.
Effusion cooling is provided by an array of small holes positioned in the combustor liner wall. A supply of cooling air is passed through the holes from the cooler surface of the combustor liner to the surface exposed to higher temperatures. The cooling air actively cools the wall by convection as it passes through the hole and film cooling after the cooling air is discharged.
The effusion holes are typically between about 0.010 and about 0.050 inches in diameter and angled so that the centerline of the hole forms a 15 to 30 degree angle with respect to the surface of the wall. This small angle increases the length of the hole through the wall thus increasing the surface area from which the cooling air can extract heat from the wall material. The small angle also allows the cooling air to enter the combustor nearly parallel to the wall surface so that a cooling film is generated on the inside of the combustor liner.
Impingement cooling is provided by an impingement plate positioned on the cooler side of the combustor wall. The impingement plate has an array of small holes there through and is spaced at a distance from the surface of the wall to provide a gap. A supply of cooling air is passed through the holes to impinge the surface of the wall and thereby extract heat from the wall material. The surface heat transfer patterns can be controlled by the configuration of the hole array to provide additional cooling to hot spots.
It is well know to the industry that combining the two methods (impingement-effusion) can provide significant improvements in cooling efficiency over either method alone. It has been reported that the heat/mass transfer rate for impingement-effusion cooling is approximately 45% to 55% higher than that for impingement cooling alone and about three to four times that for effusion cooling alone. Although the quantitative results may vary with experimental set-up and application, combined impingement-effusion cooling systems may be more efficient than either alone for some applications.
In U.S. Pat. No. 4,695,247, an impingement-effusion method is described. The disclosed combustor is a double wall construction that uses pin fins to provide spacing between the inner and outer walls. The inner wall is provided with effusion holes and the outer wall is provided with impingement holes. Because the inner wall is exposed to higher temperatures than the outer wall, the thermal growth difference between the two walls results in high thermal stresses and poor control of the impingement gap. The high thermal stresses reduce combustor life and the poor gap control reduces impingement cooling efficiency.
A method for reducing the thermal stresses is described in U.S. Pat. No. 6,237,344. The disclosed impingement baffle has integrally formed dimples. The thermal growth difference between the baffle and the hot wall is alleviated by allowing the gap to float. Unfortunately, this also results in gap variations. The impingement efficiency is very sensitive to the gap between the wall and the impingement plate, which is difficult to control because of the difference in radial thermal growth between the impingement plate and the wall. In smaller cavities such as vanes this gap difference is acceptable, but in large diameter combustor walls the gap variation is a significant sacrifice in cooling efficiency.
Many known impingement-effusion methods have included rigidly attaching two structural walls to one another. The rigidly attached hot and cold structural walls result in high thermal stresses and component life limitations. To avoid these stresses, other methods have included floating one of the walls. Floating one of the walls requires some type of seal and the cooling efficiency is very sensitive to leakage that is present in most seals.
A method that does not require floating a wall or rigidly attaching two structural walls is described In U.S. Pat. No. 5,216,886. This method attaches an array of walled liner cells to a liner support structure. Cooling air enters the cells through impingement holes in the liner support. The cooling air exits the cells through holes in the side wall portions, entering the gaps between the cells, or exits the cells through holes in the top portions of the liner cells. The cooling air then sets up a cooling air film across the top portion of the cells. Although this method may avoid the thermal stress and leakage problems mentioned above, it requires multiple walled liner cells, which increases the surface area exposed to the hot combustion gas flow. Additionally, each liner cell must have sharp edges to comply with the hot side flow path, complicating liner manufacturing. Moreover, because the cells expand axially and laterally into the gaps to alleviate the thermal stresses, variations in the momentum of the cooling air that passes through the gaps may result in cooling film disruptions for some applications. Further, a combustor liner comprising multiple liner cells may not be suitable for thermal barrier coating (TBC) applications. Conventional techniques for applying TBC, such as plasma spray, may result in TBC being deposited in the gaps between the liner cells, which in turn may disrupt the flow of cooling air or close the gap that is required for thermal expansion.
As can be seen, there is a need for a method of attaching an impingement plate directly to the combustor in such a manner that will result in acceptable stresses but will have no leakage. Further, there is a need for a method of cooling a combustor liner by impingement-effusion that allows for conventional TBC application. An impingement effusion method is needed wherein thermal stress is relieved without increasing the surface area that is exposed to the high temperature gas flow.